J. Org. Chem. 1987,52, 3691-3693
Hydrogen Abstractions from Thiophenols by Benzoyloxy Sung So0 Kim,* Jin Seog Seo, and Moon Hwan Yoon
Scheme I (Ph COO)? PhC02' t kd
Ph'
Received December 15, 1986
Results a n d Discussion Thermolyses of thiophenols with BPg in chlorobenzene gave benzoic acid, benzene, and phenyl disulfides as the sole major products showing excellent material balances for all runs.l0 Low concentrations of BP (5 X M) and relatively high reactivities" of thiophenols a t the concentrations can efficiently prevent induced and other undesirable reactions. Formationsg of diphenyl and phenyl benzoate, both derived from in- and out-of-cage recombination, and chlorodiphenyls from the addition of phenyl radicals to the solvent were not observed a t all for all reactions except for those of thiophenol, in which less than 1% formation of diphenyl, based on the quantity of B P decomposed, was detected plausibly because of its viscosity. Control experiments also indicate that occurrence of phenyl disulfides12 cannot disturb the scheme proposed. Benzoyloxy radicals may thus abstract hydrogens from thiophenols and undergo decarboxylation to give phenyl radicals, which again exclusively abstract hydrogens. Such reactions can be commonly involved with thiophenols as shown in Scheme I. Assuming steady-state concentration for phenyl radicals, we may derive eq 5. Reactions of variable concentrations [PhCOzH] = k,/k,[YPhSH] [PhHI of each thiophenol with BP in chlorobenzene led respectively t~ corresponding formations of benzene and benzoic acid. Applying eq 5, plots of molar ratios of benzoic acid to benzene against the corresponding thiophenol concentrations gave straight lines with the slopes equal to k,/kd.13 The rates thus obtained were fitted into the Hammett correlations which are shown in Table I and Figure 1. A better correlation is found by using u+ and should accordingly imply dominence of polar TS 1, exhibiting p = -1.19. This is a well-known phenomenon for hydrogen abstractions from toluenes14and analogous substrates.'""
1
2
The polar substituent effectla was first rationalized by Russelll9ascribing the polarity to the electron affiiity and the heats of reaction. The polar concept was once critiDedicated to Professor Cheves Walling on the occasion of his 71st birthday.
0022-3263/87/1952-3691$01.50/0
YPhSH
1
Department of Chemistry, Inha University, Incheon 160, South Korea
Benzoyloxy radicals derived from benzoyl peroxide (BPI may either abstract hydrogens from substrates or add to double bond^.^^^ The former reaction was rarelg investigated, while the latter has received much attention. Walling and bars studied hydrogen abstractions by benzoyloxy radicals from substrates having C-H bonds of various kinds. They compared relative rates of hydrogen abstraction using decarboxylation of benzoyloxy radical'd as a standard. We now report relative reactivities of thiophenols toward benzoyloxy radicals and rationalize the results in terms of a polar transition state (TS).
369 1
t YPhSH 2YPhS'
ha
*C
2PhC02'
(1)
PhC02H t YPhS' (2)
PhH t YPhS'
(3)
( YPh 9,
(4 )
Y: p-OCH3, p-CH3, H . p - C l , p - F
Table I. Relative Reactivities of Thiophenols toward Benzoyloxy Radicals a n d the Hammett Correlations in Chlorobenzene at 100 OC substd thiophenols' kalkdb re1 k ,
V-OCH, p-CH3 -
155.6 40.0 22.8 17.6 14.1
P-F H p-c1
8.84 2.27 1.30
1 0.80
Hammett Correlationscpd r = 0.999 r = 0.894
p(u+) = -1.19 p ( u ) = -1.91
'For the ranges of concentration of thiophenols, refer to Experimental Section. bError limits are less than 4%, being average deviations of more than three runs. c u and u+ were taken from: Ritchie, C. D.; Sager, W. F. Prog. Phys. Org. Chem. 1964, 2, 334. r, correlation coefficient.
-1.0 -1.0
I
-0.5
I
I
0.0
0.5
I
1.0
o+ ( a ) or o (0)
Figure 1. Hammett correlations for hydrogen abstractions from thiophenols by benzoyloxy radicals in chlorobenzene a t 100 "C.
cized20 but further advocated21 for in terms of the thermoneutrality and the polarizability. Recently we14 and (1) This work has been presented at the 192nd National Meeting of the American Chemical Society, Anaheim, California, September 1986; paper ORGN 239. (2)We deeply thank the reviewer for his enlightening comments on the manuscript. (3)Scaiano, J. C.;Howard, J. A. knzdolt-Bornstein. New Series. Radical Reaction Rates in Liquids; Fisher, H., Ed.; Springer-Verlag: Berlin, 1984; Vol. 13. (4) Moad, G.; Rizzardo, E.; Solomon, D. H. A u t , J. Chem. 1983,36, 1573. (5)Huang, R. L.; Lee, H. H.; Ong, S. H. J. Chem. SOC.1962, 3336. (6) Walling, C.; Azar, J. C. J. Org. Chem. 1968, 33, 3885. (7)Grossi, L.; Lusztyk, J.; Ingold, K. U. J. Org. Chem. 1985,50, 5882 and references cited therein. (8)Yamauchi, S.; Hirota, N.; Takahara, S.; Sakuragi, H.; Tokumaru, K. J. Am. Chem. SOC.1986,107, 5021. (9) DeTar, D. F. J. Am. Chem. SOC.1967,89,4058.
0 1987 American Chemical Society
3692 J. Org. Chem., Vol. 52, No. 16, 1987
others22recognize the importance of entropic term for the rates, which is mainly derived from t r a n ~ l a t i o n s lof ~ *the ~~ activated complex.24 The p(u+) = -1.19 a t 100 "C may be comparable to p ( a + ) = -0.9 a t 22 O C for hydrogen abstractions from phenols by tert-butoxy radi~als,'~ and p( a+) = -1.04 at 30 "C for the p-scissions of tertiary alkoxy radicals.25 These relatively large figures may not be fully understood by the Russell's termslg alone. We may also note the @-scissionscan gain remarkable translational entropies during the activation. Comparable polar effects were observed with hydrogen abstractions from toluenes by bromine atoms, p(u+) = -1.40 at 80 0C,14,26 and by thiyl radicals, p(u+) = -1.80 a t 80 0C,27and attributed to the polarizabilities28 of attacking radicals.27 Polarizabilities of the hydrogen donor and attacking radical were shown to alleviate activation energies for hydrogen abstractions.29 Hydrogen abstractions from 4-methyl-l-phenyl-4-(4-substituted-pheny1)pentanesby trichloromethyl radicals gave p ( u ) = -0.40 at 70 "C. The abstractions may be preceded by complexation of the radicals with the phenyl ring.30 Entropy control of reactivity of addition of arylhalo(10)The molar ratios of benzoic acid plus benzene to BP were always 2.0 f 0.05 for all runs, indicating the hydrogen abstractions from thiophenols as the sole sinks for benzoyloxy and phenyl radicals. In case of thiophenol, the ratio was BP/(benzoic acid + benzene)/phenyl disulfide = 1:(2.0 f 0.05):(0.97 f 0.04). Phenyl disulfides were not positively identified for the reactions of substituted thiophenols but tentatively assigned for by GLC retention time comparison. The patterns of GLC chromatograms were basically similar to those for reactions of thiophenol only with the difference of retention times of phenyl disulfdes. The ratio of (2.0f 0.05)may thus validate Scheme I for all reactions. We estimateM k, for PhSH
+ PhCOz'
PhCOzH + PhS'
to be 1.13 X lo6
